The Science of Forming



Interesting Details About Coefficient Of Friction

By: Stuart Keeler

Monday, September 01, 2008
A hot topic today is the rated miles per gallon (mpg) for each car model. A driver brags that he ran his own test and achieved 31 mpg in city driving —way above the 25 mpg standard rating for his model. Should we be amazed at his result? No, because insufficient data are available to interpret the conditions under which he ran his test. Three major factors control his mpg:
Fig. 1 Deformation mode and Friction test
Fig. 1—A typical stamping has five different deformation modes requiring five different friction tests to obtain appropriate coefficient of friction values.

1) the vehicle, 2) the driver and 3) the test environment.

For our discussion, let us ignore the vehicle. The second factor—the driver —obviously is very important. Does the driver start and stop slowly in anticipation of the traffic ahead or perform the stomp-on-the-accelerator slam-on-the-brakes sequence? The third factor is critical. Does the test occur between midnight and 4 a.m. or in the middle of morning rush hour? For rush hour, is the driver jammed on the expressway with all of the other traffic heading toward the town center or smoothly sailing in the opposite direction on the open road? The worst driving test is stop-and-go through a business district with nonsynchronized traffic lights.

What is the relationship between testing for mpg and testing for coefficients of friction (COF)? Assume someone tells you his lubricant has a COF of 0.1 compared to another lubricant with a COF of 0.15. That number has no meaning because a lubricant by itself has no COF. Like the mpg test, three major factors control the COF: 1) the lubricant, 2) the other two components that create a COF and 3) the test environment. Let us ignore the details of the lubricant as we did with the vehicle in the example above.

The COF is the output of three inputs acting together—the sheetmetal, the lubricant and the die. Changing one or more inputs creates a new COF. Substituting an electrogalvanized coating for a hot-dipped galvanized coating can result in a large change in COF—either an increase or decrease. Because standardized sheetmetal samples and dies are not available, the most reliable COF data are comparisons of multiple samples of different lubricants (or different sheet alloys with the same lubricant) tested at the same time in random sequence. Testing one lubricant on one day and then testing a second lubricant on some other day is not experimentally correct. Changes in the test conditions, such as ambient temperature and humidity, adjustments to test equipment, or even operators can bias the results.

Fig. 2 The draw bead simulator
Fig. 2—The Draw Bead Simulator (DBS) test requires that a test strip be pulled through fixed beads (shown in schematic) and then a second test strip pulled through replacement roller beads.
The third and key factor, however, is the test environment or type of test equipment. Many people assume that a standard test instrument exists to perform COF measurements, just as there is the tensile-test machine to measure mechanical properties. Fortunately or unfortunately, many different types of equipment are available to perform COF measurements.
Fig. 3 Changeable radius
Fig. 3—The Bending Under Tension (BUT) test pulls the strip over a radius with a back tension to duplicate binder-restraining forces.

A number of years ago, a group of metalforming specialists from a number of steel supplier and user companies met informally to formulate a project to determine any correlation of COF among a number of lubricant types and steels (bare and coated). The group quickly realized that different parts of a forming die subjected the steel and lubricants to different modes of forming. Fig. 1 shows the five different areas of the stamping, the type of forming mode for each, and the COF test equipment utilized in the test program.

The Strip Pull test is the standard high-school physics COF. A strip of metal slides through an upper and lower die without any deformation imposed. COF is the ratio of the pulling force divided by normal force imposed on the strip.

Dr. Harmon Nine of GM Research designed the original Draw Bead Simulator (DBS). The test has three solid beads—two fixed shoulder beads and one movable center bead that sets the percent of interleaf (Fig. 2). Pulling a test strip through this bead configuration determines the total force for bend/unbend deformation plus friction. When the roller beads replace the solid beads and a second strip is tested, the pulling force now is only the bend/ unbend force. Subtracting the two values provides the frictional force.

Fig. 4 The punch radius friction test
Fig. 4—The Punch Radius Friction test uses extensometers on two legs of the U-shaped specimen for computing the coefficient of friction around the radius R.

The Die Radius Friction test (now known as the Bend Under Tension or BUT test) is commonly used today (Fig. 3). The BUT has the advantage over the DBS test because a controlled back tension duplicates different amounts of blankholder restraint as the strip flows over the die radius.

The Punch Radius test simulates sheetmetal sliding around the radius between a flat bottom cup and a vertical wall (Fig. 4). Strip motion is slow compared to the DBS and BUT tests.

Finally, the Hemispherical Rotation test is a unique test designed by Dr. Nine. A hemispherical punch forms a partial dome and then stops deformation without releasing the forming force. The punch then rotates while force gauges measure the torque required to maintain movement. The torque determines the COF.

The interesting, unpublished test results from the above study and other research projects provide a useful insight about friction and its role in sheetmetal forming. We’ll discuss this in next month’s column. MF


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